Gordon Brown became the first world leader to fly into Copenhagen last night where he warned there was a possibility the climate change talks may not end in agreement.

The prime minister is attending the talks two days earlier than planned in order to help broker discussions on who should pay to tackle global warming. But he immediately cast a gloomy pall, telling reporters in Copenhagen: "It is possible that we will not get an agreement and it is also true that there are many issues to be sorted out."

With the talks balanced on a knife-edge, earlier in the day the prime minister had issued a call to arms to fellow heads of state, saying they had three days to "shape the future of humanity". As the high-level political part of the summit began, its Danish president, Connie Hedegaard, echoed the pessimism of the prime minister: "In these very hours we are balancing between success and failure. Success is within reach. But I must also warn you: we can fail."

Delegates at the summit are nervously awaiting fresh versions of the draft treaty text which more than 115 world leaders will want to finalise by Friday.

The first sign of progress could come today with Ethiopia's prime minister, Meles Zenawi, expected to announce new proposals for climate change. Developing countries say they need billions of dollars to cope with rising sea levels and extreme temperatures, and to move to clean energy. Meles, who represents the Africa group of nations, met Brown in London yesterday. Brown said Meles's proposals were an important step forward and his ideas were a "framework within which developed and developing countries can work together".

The prime minister will hope for some political credit at home from a display of energetic diplomacy at Copenhagen, believing his political strengths are best demonstrated when brokering international deals.

Meles also spoke to Barack Obama and Nicolas Sarkozy today, but he could face a backlash from some developing countries if the proposals are seen as too weak. After meeting Sarkozy, Meles said there was "near-total understanding" with the EU. The African bloc has maintained that rich nations caused the climate crisis and should pay for the damage that is occurring disproportionately on their continent.

Britain and a number of governments have been working to try to sweeten a climate change deal by increasing the finance on offer. Among the likely components of a deal is a plan from Norway and Mexico that envisages drawing on a mix of private and government funds to funnel up to $40bn (£25bn) a year to developing countries. This would be far more ambitious than the short-term fund of $10bn a year currently on the table and would help bridge the critical gap to the $100bn a year by 2020 suggested by Brown and the EU.

"It is absolutely necessary to get the scale above $10bn. The developing countries will not sign on to anything if we do not have a scaled up plan for climate funding," said Camilla Froyn, an official at the Norwegian ministry of finance, who helped draft the proposal.

The proposed fund would go into operation from 2013, and would rely on government money – from all but the poorest countries – as well as proceeds from the carbon trading market.

The Mexico-Norway plan is backed by Britain and other countries and the US is also looking favourably at the idea.

But the poorer countries fear the carbon market is volatile may not provide a steady source of funds.

India, China and Brazil have ruled out mandatory contributions to a climate fund. Officials from those countries, plus South Africa, spoke out angrily yesterday about being pressured to sign up to a deal dictated by rich countries.

"No one wants to be the country to be accused of collapsing the talks. But we fear that a political statement that is contrary to our interests may be imposed without real consultation," said one source close to the talks.

Today will also be the biggest day of demonstrations yet in Copenhagen, and the highest risk of disorder.

Thousands of activists have pledged non-violent direct action, including a Reclaim Power plan to invade the conference centre and hold a "people's assembly".